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Inorg. Chem. 1998, 37, 6942-6946
Sulfur Site Iodine Adduct of a Nickel Thiolate Complex Erica J. Lyon, Ghezai Musie, Joseph H. Reibenspies, and Marcetta Y. Darensbourg* Department of Chemistry, Texas A&M University College Station, Texas 77843 ReceiVed July 21, 1998 Introduction The metal-bound thiolates of N,N′-bis(mercaptoethyl)-1,5diazacyclooctanenickel(II), Ni-1, and N,N′-bis(2-mercaptomethylpropane)-1,5-diazacyclooctanenickel(II), Ni-1*, exhibit sulfur-based nucleophilicity ranging from alkylation,1 oxygenation,2 and metalation3 to small molecule adduct formation.4 Adducts formed from SO2 and the nickel thiolates precipitate out of methanol, forming highly stable crystalline lattices in the case of Ni-1‚SO2, Figure 1.4 Whereas a purge of N2 easily removes the SO2 from Ni-1‚SO2, dissolved in CH3CN, solid samples are stable to vacuum (∼0.01 Torr) overnight. In contrast, SO2 dissociates readily from solid and solution forms of the sterically hindered Ni-1*‚SO2. The reactivity of O2 with Ni-1 and Ni-1* is also well established. In fact, such an Ni-1‚O2 or Ni-1*‚O2 adduct is a key proposed intermediate in thiolate-S oxygenation by molecular O2 in either 1∆O2 or 3∆O2 form, a reaction that yields primarily S-bound sulfinates and sulfenates.2 An interesting aspect of the SO2 adducts is their reactivity with O2; a rapid reaction of Ni-1‚SO2 with O2 results in formation of SO42-, using oxidation of the thiolates to disulfide as the electron source in the sulfate-forming reaction.4 As implied in eq 1, and in
analogy to a similar sulfate-forming reaction by Vaska’s salt,5 we have suggested that the SO2 intercepts the sulfperoxy intermediate in the oxygenation process of Ni-SR. It is expected that such first-coordination sphere reactivity could be a hallmark (1) (a) Mills, D. K.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1990, 29, 4364. (b) Buonomo, R. M.; Reibenspies, J. H.; Darensbourg, M. Y. Chem. Ber. 1996, 129, 779. (c) Goodman, D. C.; Farmer, P. J.; Darensbourg, M. Y.; Reibenspies, J. H. Inorg. Chem. 1996, 35, 4989. (d) Goodman, D. C.; Buonomo, R. M.; Farmer, P. J.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1996, 35, 4029. (2) (a) Grapperhaus, C. A.; Darensbourg, M. Y. Acc. Chem. Res. 1998, 31, 451 and references therein. (b) Grapperhaus, C. A.; Maguire, M. J.; Tuntulani, T.; Darensbourg, M. Y. Inorg. Chem. 1997, 36, 1860. (3) (a) Musie, G.; Farmer P. J.; Tuntulani, T.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1996, 35, 2176. (b) Tuntulani, T.; Reibenspies, J. H.; Farmer, P. J.; Darensbourg, M. Y. Inorg. Chem. 1992, 31, 3497. (c) Mills, D. K.; Hsiao, Y. M.; Farmer, P. J.; Atnip, E. V.; Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 1991, 113, 1421. (4) (a) Darensbourg, M. Y.; Tuntulani, T.; Reibenspies, J. H. Inorg. Chem. 1994, 33, 611. (b) Darensbourg, M. Y.; Tuntulani, T.; Reibenspies, J. H. Inorg. Chem. 1995, 34, 6287. (5) Valentine, J.; Valentine, D., Jr.; Collman, J. P. Inorg. Chem. 1971, 10, 219.
Figure 1. A portion of the packing diagram of the crystal structure of Ni-1‚SO2.4 The shorter Sthiolate-SSO2 distance is 2.597 Å and the longer (intermolecular interaction) is 3.692 Å.
of nickel thiolates, and therefore, adducts of other diatomics might lend veracity to the proposed intermediate. Herein we report on the interactions of I2 with the nickel thiolates. The resulting adduct of the Ni-1* dithiolate may be regarded as a charge-transfer complex of diiodine, which further emphasizes that the donor ability of nickel thiolate sulfur is much like that of thioethers or triphenylphosphine.6,7 Experimental Section General Methods. Reagent-grade solvents were dried and purified according to published procedures.8 Where necessary, standard Schlenk and glovebox techniques were employed which used argon (passed through a drying column) as the inert gas. Physical Measurements. UV-vis spectra were recorded on a Hewlett-Packard HP8452A diode array spectrophotometer. Mass spectra were obtained using positive ion electrospray ionization (ESI) on a Vestec 201A quadrupole mass spectrometer. The mass spectra were recorded on a Technivent Vector One data system. Conductance measurements were performed using an Orion model 160 conductance meter equipped with an Orion two-electrode conductivity cell manufactured by Sybron Corp. The cell constant was determined to be 0.112 cm-1. X-ray crystallographic data were obtained on a Siemens R3m/V single-crystal X-ray diffractometer operating at 55 kV and 30 mA, Mo KR (λ ) 0.710 73 Å) radiation equipped with a Siemens LT-2 cryostat. Diffractometer control software P3VAX 3.42 was supplied by Siemens Analytical Instruments, Inc. All crystallographic calculations were performed with use of the Siemens SHELXTL-PLUS program package.9 The structures were solved by direct methods. Anisotropic (6) Blake, A. J.; Cristiani, F.; Devillanova, F. A.; Garau, A.; Gilby, L. M.; Gould, R. O.; Isaia, F.; Lippolis, V.; Parsons, S.; Radek, C.; Schro¨der, M. J. Chem. Soc., Dalton Trans. 1997, 1337. (7) Cotton, F. A.; Kibala, P. A. J. Am. Chem. Soc. 1987, 109, 3308. (8) Gordon, A. J.; Ford, R. A. The Chemist’s Companion, Wiley and Sons: New York, 1972; p 429. (9) Sheldrick, G. SHELXTL-PLUS Program for Crystal Structure Refinement; Institu¨t fu¨r Anorganische Chemie der Universita¨t: Gottingen, Germany, 1990. (10) Mills, D. K.; Font, I.; Farmer, P. J.; Tuntulani, T.; Buonomo, R. M.; Goodman, D. C.; Musie, G.; Grapperhaus, C. A.; Maguire, M. J.; Lai, C.-H.; Hatley, M. L.; Smee, J. J.; Bellefeuille, J. A.; Darensbourg, M. Y. Inorganic Synthesis; Wiley and Sons: New York, 1998; Vol. 32, p 82.
10.1021/ic980852o CCC: $15.00 © 1998 American Chemical Society Published on Web 12/03/1998
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Inorganic Chemistry, Vol. 37, No. 26, 1998 6943
Table 1. Crystal Parameters and X-ray Diffraction Data for Ni-1*‚I2 empirical formula fw space group a, Å b, Å c, Å β, deg V, Å3 F (calcd) g/cm3 Z temp, °C radiation (λ, Å) min/max transm abs coeff, cm-1 R(F),a Rw(F2)b
C14H28N2S2I2Ni 601.01 P21/c 7.826(3) 22.292(8) 12.430(5) 108.01(3) 2062.2(13) 1.936 4 25 Mo KR (0.710 73) 0.803/0.997 41.34 0.0652, 0.1300
R(F) ) ∑|Fo - Fc|/∑|Fo|. Rw(F2) ) {[∑w(Fo2 - Fc2)2]/ [∑w(Fo2)2]}1/2. b I > 2σ(I). a
Figure 2. Molecular structure of Ni-1*‚I2 with hydrogens omitted; thermal ellipsoids represent 50% probability.
Results and Discussion refinement for all non-hydrogen atoms was done by a full-matrix leastsquares method. A single crystal was mounted on a glass fiber with epoxy cement either at room temperature or at 163 K in an N2 cold stream. Synthesis and Reactions. Complexes N,N′-bis(mercaptoethyl)-1,5diazacyclooctanenickel(II), Ni-1, and N,N′-bis(2-mercaptomethylpropane)-1,5-diazacyclooctanenickel(II), Ni-1*, were prepared according to published procedures.10 Ni-1 + I2. In a 50-mL Schlenk flask, 50 mg (0.172 mmol) of Ni-1 was degassed and dissolved in 3 mL of MeOH. To this solution, I2 (44 mg, 0.173 mmol dissolved in 5 mL of MeOH) was added via cannula without stirring. After 3 min, a fine brown powder began to settle out. Within 10 minutes, the solution was almost colorless. The supernatant was removed from the brown solid by cannula, and the product was washed with MeOH (4 × 10-mL aliquots) to remove any unreacted starting materials. Mass spectroscopy indicated the formation of a previously characterized trimetallic product, [(Ni-1)2Ni]2+(I-)2,11 obtained in ca. 75% yield. Ni-1*‚I2. A 50-mL round-bottomed flask containing 57.7 mg (0.166 mmol) of Ni-1* was degassed, and to it was added 15 mL of dry, degassed MeOH. In a separate flask, 42.8 mg (0.168 mmol) of I2 was degassed briefly to expel most of the oxygen but not long enough to have significant loss of the I2 as a vapor. The iodine was then dissolved in 10 mL of dry, degassed MeOH, and the solution was slowly transferred by cannula into the flask containing the purple Ni-1* solution. Without stirring, a fluffy microcrystalline brown precipitate settled out after 3-4 min. (Stirring affords a fine brown powder.) The supernatant was then removed via cannula, and the product was washed 4 times with 15-mL aliquots of MeOH to remove unreacted starting material. After being dried, the brown product was isolated in yields of approximately 70% and was found to be air stable for several days. This product is also stable under vacuum (∼0.01 Torr) for several days. Anal. Calcd (measured) for C14H28N2S2NiI2: C, 28.0 (27.8); H, 4.70 (4.74); N, 4.66 (4.56). Crystals were obtained using FeI2 as the iodine source. The FeI2 was weighed out in the glovebox (0.07 g/0.23 mmol) and dissolved in 40 mL of THF. The solution was stirred for 2 h and then filtered into a flask containing 0.0784 g of Ni-1* dissolved in a minimum of methanol (∼2-3 mL). Such solutions were transferred to double crystallization tubes, using ether in the outer tube for diffusion. Dark brown, air-stable crystals were selected (picked out of a mixture of brown powder and crystalline material) for X-ray crystallographic analysis and found to be in the monoclinic space group of P21/c. The cell parameters are contained in Table 1, and a full structure report is deposited as Supporting Information. (11) Farmer, P. F.; Solouki, T.; Mills, D. K.; Soma, T.; Russell, D. H.; Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 1992, 114, 4601. (12) Frey, M. Struct. Bonding 1998, 90, 97.
Iodine, I2, reacts with Ni-1 in methanol to yield the wellcharacterized trimetallic via a process attributable to electron transfer from thiolate, disulfide formation with Ni2+ loss, and subsequent attachment to two parent Ni-1 complexes, eq 2.11
In contrast, similar exposure of the sterically hindered Ni-1* to iodine yields the brown Ni-1*‚I2 adduct, eq 3. While pure I2
produced powders of the adduct which analyzed well for the 1:1 adduct, crystals suitable for X-ray analysis were obtained only when FeI2 (presumably contaminated with excess I2 as suggested by the purple vapor over the solid material) was used as the iodine source. Originally introduced to Ni-1* for the purpose of preparing thiolate-bridged NiFe heterometallics as models for the active site of [NiFe]H2ase,12 the “iodine rich” FeI2 evidently serves as a slow release source of I2. Such an observation of iodine adducts arising from metal iodides is not without precedence; for example, Cotton and Kibala observed products arising from Ph3P‚I2 interaction while examining the reactivity of Ph3P with ZrI4.7 Subsequently the Ph3P‚I2 adduct was structurally characterized, vide infra.13 Figure 2 shows the molecular structure of Ni-1*‚I2 and a portion of the packing diagram is given in Figure 3. The bond lengths and angles listed in Table 2 indicate the Ni coordination in Ni-1*‚I2 is little changed from the parent Ni-1*. Similar results were observed for the SO2 adduct of Ni-1 in that the NiN2S2 plane is conserved.4 The I2 sits on a sulfur with (13) Godfrey, S. M.; Kelly, D. G.; McAuliffe, C. A.; Mackie, A. G.; Pritchard, R. G.; Watson, S. M. J. Chem. Soc., Chem. Commun. 1991, 1163. (14) Karle, I. L. J. Chem. Phys. 1955, 23, 1739. (15) van Bolhuis, F.; Koster, P. B.; Migchelsen, T. Acta Crystallogr. 1967, 23, 940.
6944 Inorganic Chemistry, Vol. 37, No. 26, 1998
Notes
Figure 3. Packing diagram of Ni-1*‚I2 displaying the orientation of I-I with respect to the next closest molecule.
substantial linearity of the S-I-I bonds, 176.9 (1)°, as expected from previous examples of R2S-I2 adducts.6 The ∠Ni-S(2)I(1) of 101.02° suggests the involvement of a thiolate sulfur lone pair with tetrahedral character and is characterized by a relatively short S-I distance of 2.601(4) Å. This apparently strong S:fI bond gives rise to an I-I bond distance of 3.026(2) Å, which is significantly longer than the bond distances of free iodine in the gaseous state, 2.667(2) Å,14 and in the solid state, 2.715(6) Å.15 The donor/acceptor bond distance data observed for Ni-1*‚ I2 is consistent with the inverse relationship of S-I and I-I distances found for a series of structurally characterized adducts of I2 and thioethers/thiocrown ethers as analyzed by Shro¨der et al.6 Thus the S-I bonding interaction results in donation of electron density into the I2 σ* orbital, i.e., antibonding with respect to the I-I bond.16 Although elongated, the second iodine atom is still close enough to be considered bound and not a counterion.7 The extended structure or packing diagram of Ni-1*‚I2, Figure 3, shows that the iodines are oriented such that they cannot interact with other sulfurs or iodines from neighboring complexes. This is unusual for iodine complexes which typically form chains through long-range interactions.17 For example, only one of a series of thiacrown-I2 adducts, [14]aneS4‚I2, shows molecularly distinct R2S‚I2 units similar to Ni-1*‚I2.6 As a solid or in solution, Ni-1*‚I2 is stable upon exposure to air. Thermal decomposition of the solid occurs at >200 °C, and there is no evidence of I2 loss, even at the high temperatures preceding decomposition. Like the SO2 adduct, the smallmolecule adduct cannot be disrupted when the solid is placed under vacuum for extended periods of time. In contrast to the SO2 adducts, Ni-1‚SO2 or Ni-1*‚SO2, the iodine adduct is not disrupted or degraded when a CH3CN solution of Ni-1*‚I2 is purged with an inert gas. The adduct, Ni-1*‚I2, is insoluble in most organic solvents. It is slightly soluble in CH3CN (30 mg/ 100 mL) and also in the more polar solvent DMSO. However, (16) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: New York, 1984; p 940. (17) Tipton, A. L.; Lonergan, M. C.; Stern, C. L.; Shriver, D. F. Inorg. Chim. Acta 1992, 201, 23.
Table 2. Comparison of Ni-1*‚I2 metric data to the parent complex, Ni-1*
on long standing in DMSO, the orange brown color of the solution fades to colorless with no observable precipitate. The Nature of “Ni-1*‚I2” in Polar Solvents. The solution injection technique of ESI mass spectroscopy shows two nickelcontaining fragments, as established by isotopomer distribution, for Ni-1*‚I2, neither of which is the parent peak. One signal has a m/z value of 346 which corresponds to the dithiolate, Ni1*. The other peak has a m/z value of 473 which corresponds to Ni-1*‚I+, implying a heterolysis of the I2 bond. (18) (a) Fuoss, R. M.; Accascina, F. Electrolytic Conductance; Interscience Publishers: New York, 1959; pp 294-272. (b) Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry; Wiley and Sons: New York, 1991; pp 103-105.
Notes
Inorganic Chemistry, Vol. 37, No. 26, 1998 6945
Figure 4. UV-vis of (s) I2, (0) Ni-1*, (b) Ni-1*‚I2. All samples are 0.2 mM in CH3CN and 22 °C.
The concentration dependence of solution conductivity of Ni1*‚I2 provides further evidence of heterolytic I2 cleavage or ionization in solution. For both Ni-1*‚I2 and pure I2 in CH3CN, plots of molar conductivity versus concentration follow the form of a higher-order polynomial instead of a straight line, suggesting ionization with ion-pairing occurs.18 For both, the molar conductivities at a concentration of 1 mM are approximately 160, which corresponds to a uni-uni electrolyte in solution, attributable to Ni-1*‚I+/I- for the former and I+/ I3- for the latter. Figure 4 presents the UV-vis spectrum (in CH3CN) of the Ni-1*‚I2 adduct with spectral overlays of pure I2 and Ni-1* for comparison. All are at 0.2 mM concentration. Notably, the absorbances at 362 and 292 nm are observed for both I2 and Ni-1*‚I2, with a large gain in intensity for the latter. Since the conductivity and mass spectral data above suggest heterolytic cleavage of I2, we assume the UV-vis spectrum indicates the presence of I3- and Ni-1*‚I+ in the polar solvent. More I3- is produced in solution of the Ni-1*‚I2 adduct than in pure I2. The L f M charge-transfer band at 280 nm for Ni-1* appears to shift into and be overwhelmed by the 292-nm band of I2 in the spectrum of the Ni-1*‚I2 adduct. Since the Ni-1*‚I+ species has not yet been isolated in the absence of the I-/I2/I3- equilibria, the nature of the charge-transfer band is not clearly established. While there are numerous examples of I2 adducts of organic sulfides, R2S‚‚‚I2,6,17,19 and thiones, R2CdS‚‚‚I2,20 and a few cases of coordination complex inorganic sulfides using a bridging sulfide as the I2 binding site,21 two adducts appear to be most pertinent as analogues of Ni-1*‚I2. The structure of the [14]aneS4‚I2 tetrathiacrown ether adduct, vide supra,6 is appropriate for comparison because of its lack of intermolecular interactions. It shows a longer S-I distance, 2.859(3) Å, and a shorter I-I distance, 2.8095(11) Å, as compared to the Ni-1*‚ I2 adduct, thus indicating the greater donor ability of the formally anionic thiolato sulfur as compared to the organic thioether. This result correlates with earlier studies as described below. Earlier we assembled data for a comparison of the ability of the Ni-1 dithiolate with thioethers, thiolates, and the classic (19) Herbstein, F. H.; Ashkenazi, P.; Kaftory, M.; Kapon, M.; Reisner, G. M.; Ginsburg, D. Acta Crystallogr. 1986, B42, 575. (20) (a) Freeman, F.; Ziller, J. W.; Po, H. N.; Keindl, M. C. J. Am. Chem. Soc. 1988, 110, 2586. (b) Bigoli, F.; Deplano, P.; Mercuri, M. L.; Pellinghelli, M. A.; Trogu, E. F. Phosporus, Sulfur Silicon Relat. Elem. 1992, 70, 175. (c) Bigoli, F.; Deplano, P.; Mercuri, M. L.; Pellinghelli, M. A.; Sabatini, A.; Trogu, E. F.; Vacca, A. J. Chem. Soc., Dalton Trans. 1996, 17, 3583. (d) Lu, F. L.; Keshavarz-K., M.; Srdanov, G.; Jacobson, R. H.; Wudl, F. J. Org. Chem. 1989, 54, 2165. (e) Cristiani, F.; Devillanova, F. A.; Isaia, F.; Lippolis, V.; Verani, G.; Demartin, F. Polyhedron 1995, 14, 2937. (21) (a) Allshouse, J.; Haltiwanger, R. C.; Allured, V.; DuBois, M. R. Inorg. Chem. 1994, 33, 2505. (b) Lee, J. Q.; Sampson, M. L.; Richardson, J. F.; Noble, M. E. Inorg. Chem. 1995, 34, 5055.
Figure 5. Comparison of nickel thiolate reactivity as nucleophile and analogous triphenylphosphine products.
ligand in organometallic chemistry, Ph3P, to serve as a 2-electron donor ligand to Fe(CO)4 using ν(CO) IR spectroscopy as a probe.22 The ν(CO) values are listed in footnote 2322,24,25 and indicate an order of donor ability: SPh- g Ni‚1 > PPh3 = MeSPh. Thus the S-I and I-I distances in I2 adducts of Ni-1* and the thiacrowns are consistent with their donor abilities to the metal carbonyl. However, the interaction with PPh3 is not so clear-cut. As indicated above, a report of I2 reactions with PPh3,7 which included disproportionation of I2 to yield [PPh3I]+I3-, predated a report of the simple adduct Ph3P‚I2 whose X-ray crystal structure finds a quite long I-I distance of 3.16 Å.13 This apparent reversal of donor ability of PPh3 and Ni-1* toward I2 as contrasted to their donor ability toward Fe(CO)4 must be attributed to the nature of the acceptors. Unlike I2, the metal carbonyl Fe(CO)4 has capability to back-donate electron density to π-accepting ligands. The metal thiolate has no possibility of π-accepting; PPh3 has at least a limited capability. Figure 5 outlines further analogies of the P-donor nucleophile PPh3 to the S-donor of Ni-1*, including oxygenation,2 alkylation,1 and ligation.22 A conspicuous deviation in reaction possibilities of PPh3 and Ni-1 is the potential for redox activity in the latter. The sulfur-based electron transfer from Ni-1 which oxidizes the thiolate to thiyl and hence to disulfide and reduces I2 to I- is obviously impossible for PPh3. Since Ni-1 and Ni-1* have similar and accessible Eox values, a disparity is found in that electron transfer from Ni-1 leads to the fully oxidized trimetallic complex, while this does not occur with the Ni-1* species. The cause of the discrepancy in Ni-1 and Ni-1* reactivity, eqs 2 and 3, must lie in the relative stability of the products. The trimetallic of Ni-1 is a ubiquitous product of oxidation, whereas the steric hindrance in Ni-1* greatly impedes formation of aggregates. (22) Lai, C.-H.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 2390. (23) Solution ν(CO) IR (cm-1) for LFe(CO)4: L ) Ni-1 (THF) 2030 (s), 1945 (m), 1926 (vs), 1907 (vs);22 PPh3 (hexane): 2052 (m), 1980 (m), 1947 (vs);24 MeSPh (hexane): 2057 (m), 1979 (s), 1956 (vs), 1946 (vs);25 SPh- (acetone), 2016 (m), 1910 (vs).25 (24) (a) Darensbourg, D. J.; Nelson, H. H.; Hyde, C. L. Inorg. Chem. 1974, 13, 2135-2145. (b) Darensbourg, D. J. Inorg. Chim. Acta 1970, 4, 597-601. (25) Liaw, W.-F.; Kim, C.; Darensbourg, M. Y.; Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 3591-3597.
6946 Inorganic Chemistry, Vol. 37, No. 26, 1998 Acknowledgment. Financial support from the National Science Foundation (CHE 94-15901 and CHE 98-12355) for this work and CHE 85-13273 for the X-ray diffractometer and crystallographic computing system is acknowledged. Contributions from the R. A. Welch Foundation are also gratefully acknowledged.
Notes Supporting Information Available: Diagrams and tables of crystallographic data collection parameters, atomic coordinates, and a complete listing of bond lengths and bond angles for Ni-1*‚I2 (6 pages). Ordering information is given on any current masthead page. IC980852O
